Various detectors are known. Some are configured for smoke detection. Others are useful for detecting toxic gases, for example. Many such detectors use ionization for detecting the presence or amount of the substance or gas of interest.
One known type of smoke detector includes Americium 241 as a source of alpha particles used for ionizing air. Alpha particles admitted from the Americium 241 pass into the smoke detector internal space where they ionize the air within that space. When smoke enters into the internal space, the smoke interacts with the ions and alters the ion concentration and distribution within the internal space. This can be detected, for example, by measuring the voltage or current at a collector electrode of the smoke detector.
Another known type of smoke detector utilizes light for detecting smoke particles. A light source emits light into the smoke detector internal space. A photo-sensitive detector is positioned to receive some or none of the light emitted from the light source. When smoke enters into the internal space, the smoke absorbs and scatters light from the light source. This can be detected, for example, by measuring scattered light intensity or a change in light intensity.
There have been attempts to combine ionization and photoelectric smoke detectors to provide both technologies in a single housing. For example, U.S. Pat. No. 5,633,501 describes a smoke detector that uses a source of light for photoelectric smoke detection and a source of ions for ionizing smoke detection within the same unit.